Raw copper produced by smelting is frequently later electrolytically refined. For this reason, the molten copper product must be suitable for casting anodes. This makes it necessary to refine the copper sufficiently to remove substantial quantities of dissolved sulphur and oxygen in order to cast desirable anodes. If the metal were cast directly into anodes, the high level of sulphur (typically about 0.05%) and dissolved oxygen (typically about 0.5%) would combine to form SO.sub.2 blisters in the cast metal. The purpose of in-line refining, as practiced with the present invention, is primarialy to remove sulphur from the molten copper. This is done in two steps: "blowing" and "poling". Blowing is the oxygenating step, wherein the sulphur is oxidized (to gaseous SO.sub.2), lowering the sulphur level (to typically 0.003%) in the molten copper. Poling, the introducing of a hydrocarbon into the melt, minimizes the formation of copper oxide during solidification.
The present invention is directed to improvements in apparatus for the oxidation of molten copper. In this process for in-line copper refining, a stream of molten copper from a furnace is subjected to oxygenation as it passes through one of a series of vessels during a continuous refining process.
At first, experiments were attempted useing an oxygen lance similar to those used in the steel industry to direct gas onto the surface of the molten copper. Insufficient oxygenation resulted; due at least in part to a slag layer floating on the surface of the molten copper. The lance was generally incapable of penetrating the slag layer.
Another method of oxygenating the copper is by passing a stream of oxygen through a porous plug into the molten metal as shown, for example, by U.S. Pat. Nos. 3,904,180; 3,917,242; 3,972,709; or 4,277,381. An experimental arrangement was undertaken to determine the feasibility of using a porous plug to bubble oxygen into the molten copper while it flowed through the vessel, and over the porous plug, by gravity. When bubbling pure oxygen through the porous plug, the plug failed prematurely. Two failure modes were noted; in the first, the pure oxygen reacted with the hot steel case of the porous plug and the plug shell melted and failed. In the second failure mode, molten copper peneterated the porous plug and, combined with the first failure mode, molten copper leaked from the apparatus. It was found, however, that the plug did not fail if oxygen and nitrogen were mixed and bubbled through a copper sheathed porous plug fed with copper piping. Unfortunately, insufficient oxygenation of the molten copper still took place.